1. Field of the Invention
The present invention relates to oriented rubber-reinforced polystyrene film that has a preferential orientation in the stretched direction and a shrink-label film comprising such a polystyrene film as well as a composition useful for making such films.
2. Description of Related Art
Shrink labels generally fall into two categories: roll-on shrink-on (ROSO) labels and sleeve-type labels; sleeve labels are also sometimes referred to as tube labels. ROSO labels are film sheets that wrap around a container. Sleeve labels are tubular in configuration and fit around a container by placement over the container, such that the container is surrounded by the tube. Application of heat to a shrink label that is around a container causes the label to shrink and conform to the container.
To conform to a container, each type of label must shrink preferentially (that is, to a greater extent than in any other direction) in the direction extending circumferentially around the container. ROSO films usually reside on a container with the machine direction (MD) of the film extending circumferentially around the container. Hence, ROSO films primarily shrink in the film's machine direction (MD) due to preferential machine direction orientation (MDO). In contrast, sleeve labels usually reside on a container with the label's transverse direction (TD) extending circumferentially around the container. Hence, sleeve labels shrink primarily in the film's transverse direction (TD) due to preferential transverse direction orientation (TDO).
While ROSO labels offer advantages in production speed, sleeve labels historically have enjoyed an advantage in extent of shrinkage around a container. Sleeve labels typically shrink up to 70 percent (%) around the circumference of a container. Sleeve labels, which either have no glue joint or have a glue joint that is extensively cured prior to application to a container, can tolerate a greater extent of stress during shrinkage.
Sleeve labels historically enjoy more extensive shrinkage and therefore have conformed better to contoured containers than ROSO labels. However, ROSO labels have a production advantage of being oriented in the machine direction, that is the direction they travel through machinery used during their production. It, therefore, desirable to identify an oriented film suitable for preparing a ROSO label that can shrink circumferentially around a container to a greater extent than polypropylene ROSO labels but preferably without the detriment of failure at the glue joint of the label.
Polystyrene (PS) is a particularly desirable polymer for shrink labels. Shrink label films of polypropylene (PP), for example, typically shrink only up to about 20% in any direction at a temperature below 120° C. The crystalline nature of PP requires heating above the PP's crystalline melt temperature to release additional orientation. In contrast, PS-based shrink label films only need to exceed the polymer's glass transition temperature (which generally is lower than PP's crystalline melt temperature) due to its amorphous character. Therefore, PS films can desirably provide greater shrink at lower processing temperatures than PP films.
Additionally, PS retains a higher surface energy after corona treatment (typically needed to render the surface of a polymer film suitable for printing) for extended periods of time relative to PP. Therefore, unlike PP films, corona treatment of PS films can occur during manufacture rather than just prior to printing into labels.
In contrast to copolyester and polyvinyl chloride (PVC) films, use of PS films facilitate bottle and label recyclability, as the lower density allows the label to be easily separated from the higher density (for example, polyester) bottles. Furthermore, the lower PS density advantageously provides a higher film yield, or more area/lb. or kg of film. Higher density labelstock, such as copolyester or PVC films, do not provide similar advantages.
Polystyrene-based shrink label films can include a high impact polystyrene (HIPS) component in order to improve label toughness (for example, tear resistance). However, rubber particles in a typical HIPS range have an average particle size of greater than one micrometer (see, for example, U.S. Pat. No. 6,897,260, column 4, lines 26-27). Large rubber particles tend to decrease clarity of a label film, interfering with the use of the film for reverse side printing (printing on the side of a label film proximate to the container so that it is readable through the film) as well as with viewing of the container or product through the label. Typical HIPS also contains greater than 7 percent rubber based on total HIPS weight. High concentrations of rubber can hinder the printability of a film, decrease clarity of a film, reduce dimensional stability and undesirably increase gel amount in a final film. However, in some situations such as small diameter bottles or bottle necks, HIPS alone may not supply sufficient toughness to avoid a tendency to split under stress.
It is desirable to have an oriented PS film that is suitable for shrink label applications. It is further desirable for the film to contain a high impact polystyrene of a type that has smaller rubber particles and lower rubber concentrations than that of typical HIPS in order to achieve film toughening without substantially hindering printability or clarity of the film. It is further desirable for the film to contain clear impact resistant polystyrene based on block copolymer technology to further improve film toughness. It is still further desirable if such a film can serve as a shrink label that demonstrates circumferential shrink around a container comparable to that achieved with PVC or polyester.